We report an experimental study on the current-voltage characteristics of organic solar cells (OSCs) under indoor light illumination. A daylight color light-emitting diode (LED) was used as the indoor light source. We investigated the short circuit current density, open circuit voltage, and fill factor of the OSC under LED irradiation and compared them with those for a crystal silicon solar cell (c-Si-SC), which occupy a large part of the solar cell market. We found that compared with the c-Si-SC, the OSC had higher power conversion efficiency (PCE). We also derived the maximum feasible PCE of an OSC for indoor applications and calculated that a PCE value of 21.3% could be obtained under daylight color LED illumination at approximately 200 lx. From the results of the calculation, it became apparent that the open circuit voltage plays an important role in achieving a high PCE from OSCs, indicating they are promising as electrical energy harvesting module for indoor applications.

The power conversion efficiency (PCE) of organic photovoltaic (OPV) modules with 9.5% (25 cm2) and 8.7% (802 cm2) have been demonstrated. This PCE of the module exceeded our previous world records of 8.5% (25 cm2) and 6.8% (396 cm2) that were listed in the latest Solar Cell Efficiency Tables ver.43 [1]. Both module design and coating/patterning technique were consistently studied for module development. In order to achieve highly efficient modules, we increased the ratio of photo-active area to designated illumination area to 94% without any scribing process and placed insulating layers in order to decrease the leakage current. The meniscus coating method was used for the fabrication of both buffer and photoactive layers. This technique ensures the fabrication of uniform and nanometer order thickness layers with thickness variation less than 3%. Furthermore, the PCE of the OPV under indoor illumination was found to be higher than that of the conventional Si type solar cells. This indicates that OPVs are promising as electrical power supplies for indoor applications. Therefore, we have also developed several prototypes for electronics integrated photovoltaics (EIPV) such as electrical shelf labels and wireless sensors embedded with our OPV modules, which can be operated by indoor lights.

We have isolated the coloring species of a leuco dye colored by three phenolic color developers and have revealed their molecular structures by X-ray crystallographic analysis for the first time: one leuco dye molecule combined with one developer molecule by the hydrogen bonds between the carboxylate anion of a leuco dye and the OH groups of a color developer.

Although the interface between poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and perovskite layers is one of the key factors to determine the open-circuit voltage of inverted perovskite solar cells, this interface remains poorly understood. We investigated the interface by electron spin resonance spectroscopy and found that the PEDOT undergoes dedoping in contact with the perovskite. The holes in PEDOT were reduced by electron transfer from the perovskite. Additional electron transfer occurred upon light irradiation. The electron transfer would be related to lower open-circuit voltages of the inverted structure compared to those of mesoporous and planar structures.